miraco
Experienced Member
Sorry about the hyphens, they’re not negative. I checked again and all four are 0.
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Intermittent CBM 8032 Power on Issue
- Thread starter miraco
- Start date
daver2
10k Member
If they are real, let's patent the discovery and become rich!
An old Commodore PET develops negative resistance at room temperature...
...
Dave
An old Commodore PET develops negative resistance at room temperature...
...Dave
miraco
Experienced Member
Ohms, K ohms or M ohms?
miraco
Experienced Member
1+ and 4-. 15
4+ and 1-. 0
5+ and 4-. 15
4+ and 5-. 0
4+ and 1-. 0
5+ and 4-. 15
4+ and 5-. 0
Last edited:
SiriusHardware
Veteran Member
It's the main power plug / connector going to the mainboard from the transformer / large offboard capacitor. Pin 4 of this pin / socket connection, which carries the unsmoothed 9V DC power out from the rectifiers to the large offboard smoothing capacitor, is especially prone to going intermittent, which in turn makes the +5V supplies come and go.
daver2
10k Member
Is the 0 actually 0 or infinity (i.e. a very high resistance)?
A diode should be about 600 Ohms when conducting and a very high resistance when reversed biassed.
Your multimeter should indicate whether the reading is in Ohms, k or M.
Try shorting the multimeter probes together, that would indicate what 0 Ohms looks like. Leave the multimeter probes disconnected, that would indicate what 'infinite' Ohms looks like.
Dave
A diode should be about 600 Ohms when conducting and a very high resistance when reversed biassed.
Your multimeter should indicate whether the reading is in Ohms, k or M.
Try shorting the multimeter probes together, that would indicate what 0 Ohms looks like. Leave the multimeter probes disconnected, that would indicate what 'infinite' Ohms looks like.
Dave
I had a similar issue in the reset circuit. If the machine sat for any length of time, a specific cap needed to be energized for a minute or so to re-form and start acting like a cap again.
It was maddening because the machine will work fine for each power up after it's reformed.
C51 on my 4032.
Hugo Holden
Veteran Member
If they are real, let's patent the discovery and become rich!
An old Commodore PET develops negative resistance at room temperature...
Dave
Not difficult at room temperature, a Tunnel Diode does it fine. It my well be one of the few Quantum Mechanically explained devices in the workshop. Others probably are too, but we don't need to explain them this way.
daver2
10k Member
@Hugo Holden,
That is cheating a bit...
There is an area of the V/I curve for a Tunnel Diode that has a negative slope - but at no time does the resistance itself ever go negative...
Dave
That is cheating a bit...
There is an area of the V/I curve for a Tunnel Diode that has a negative slope - but at no time does the resistance itself ever go negative...
Dave
miraco
Experienced Member
Sorry for the delay. I’ve not had this much trouble before measuring resistance 
I know the key on J8 is 7 so I should haue the correct pins.
The DMM in auto mode showing M ohms I see this:
1+ and 4-. 1.9
4+ and 1-. infinite (OL)
5+ and 4-. 1.9
4+ and 5-. infinite (OL)
I know the key on J8 is 7 so I should haue the correct pins.
The DMM in auto mode showing M ohms I see this:
1+ and 4-. 1.9
4+ and 1-. infinite (OL)
5+ and 4-. 1.9
4+ and 5-. infinite (OL)
daver2
10k Member
Can you just confirm that the 1.9 is 1.9 MOhms?
I prefer to take the multimeter OFF AUTO ad put it onto manual to save confusion...
1.9 MOhms sounds a lot to me... I assume you are using a digital multimeter? Does it have a diode test function by any chance?
Dave
I prefer to take the multimeter OFF AUTO ad put it onto manual to save confusion...
1.9 MOhms sounds a lot to me... I assume you are using a digital multimeter? Does it have a diode test function by any chance?
Dave
miraco
Experienced Member
Yes, I’m using a Fluke DMM Which has a diode test option. I have replaced the battery and verified using resistors I have in the parts box. I am testing the board on the bench disconnected from power.
I believe the auto function worked correctly for that test but also set manual range to measure as well. Taking the readings on the manual K range I am getting this from either side of board:
1+ and 4-. 4.1
4+ and 1-. 0L
5+ and 4-. 4.5
4+ and 5-. 0L
Don’t understand what I could be doing wrong.
I believe the auto function worked correctly for that test but also set manual range to measure as well. Taking the readings on the manual K range I am getting this from either side of board:
1+ and 4-. 4.1
4+ and 1-. 0L
5+ and 4-. 4.5
4+ and 5-. 0L
Don’t understand what I could be doing wrong.
daver2
10k Member
It may be that using the resistance range to check a diode is not sensible with your digital multimeter due to the voltage/current used.
I use a cheap and cheerful analogue multimeter when testing diodes and that is fine.
If there is a diode tester on your multimeter I would use that instead.
What Fluke are you using?
Dave
I use a cheap and cheerful analogue multimeter when testing diodes and that is fine.
If there is a diode tester on your multimeter I would use that instead.
What Fluke are you using?
Dave
miraco
Experienced Member
117 which I was lucky to pay less than full price for lol. I also have a simpler Rasio Shack multimeter that has fewer options and an older style RS analog meter.
I am going out for a couple hours but will be back home later this afternoon.
I am going out for a couple hours but will be back home later this afternoon.
daver2
10k Member
I am going out shortly as well - a bit of a time difference between us of course...
Dave
Dave
daver2
10k Member
I looked up the Fluke 117 manual.
In this case, I would use the DIODE TEST setting.
In one direction (forward biased), you should read the forward voltage drop of the diode - which should be in the range 0.6V DC to 0.7V DC. The meter should give you a single BEEP. When you reverse the probes of the multimeter on the diode, it should display as 0L (indicating a high resistance).
If you don't get a BEEP (and observe a sensible V DC reading) or get a constant BEEP - the diode we are testing (1 of 4 inside the full-wave bridge rectifier) is suspect.
Dave
In this case, I would use the DIODE TEST setting.
In one direction (forward biased), you should read the forward voltage drop of the diode - which should be in the range 0.6V DC to 0.7V DC. The meter should give you a single BEEP. When you reverse the probes of the multimeter on the diode, it should display as 0L (indicating a high resistance).
If you don't get a BEEP (and observe a sensible V DC reading) or get a constant BEEP - the diode we are testing (1 of 4 inside the full-wave bridge rectifier) is suspect.
Dave
miraco
Experienced Member
DMM set to diode test
1+ and 4-. .48VDC
4+ and 1-. 0L
5+ and 4-. .52VDC
4+ and 5-. 0L
1+ and 4-. .48VDC
4+ and 1-. 0L
5+ and 4-. .52VDC
4+ and 5-. 0L
Last edited:
daver2
10k Member
That looks better now.
I will post the next test tomorrow morning. We need to test the other half of the bridge rectifier next.
>>> Seem a bit low.
It depends whether the meter is calibrated or not I suppose. I didn't think there were too many 'sneak paths' to confuse the readings either.
Dave
I will post the next test tomorrow morning. We need to test the other half of the bridge rectifier next.
>>> Seem a bit low.
It depends whether the meter is calibrated or not I suppose. I didn't think there were too many 'sneak paths' to confuse the readings either.
Dave